In electric equipment including a solenoid valve, a motor, or an electric circuit, soft magnetic materials manufactured by powder metallurgy are used. The soft magnetic materials each include a plurality of composite magnetic particles each including a metal magnetic particle composed of, for example, pure iron, and an insulation coating composed of, for example, a phosphate, which covers the surface of the metal magnetic particle. From the requirement for improving energy conversion efficiency and decreasing heat generation, the soft magnetic materials are required to have the magnetic property that a high magnetic flux density can be obtained by applying a small magnetic field and the magnetic property that the energy loss due to a change in the magnetic flux density is small.
When a powder magnetic core formed using such a soft magnetic material is used in an AC magnetic field, an energy loss referred to as an “iron loss” occurs. The iron loss is represented by a total of a hysteresis loss and an eddy-current loss. The hysteresis loss is an energy loss produced by the energy necessary for changing the magnetic flux density of a soft magnetic material, and the eddy-current loss is an energy loss produced by an eddy current flowing between the metal magnetic particles constituting the soft magnetic material. The hysteresis loss is proportional to an operating frequency, and the eddy-current loss is proportional to the square of the operating frequency. Therefore, the hysteresis loss becomes dominant in a low frequency region, and the eddy-current loss becomes dominant mainly in a high frequency region. The powder magnetic core is required to have the magnetic property of decreasing the occurrence of an iron loss, i.e., high AC magnetic properties.
In order to decrease the hysteresis loss of the iron loss of a soft magnetic material, distortion and displacement in the metal magnetic particles may be removed to facilitate the movement of magnetic walls and decrease the coercive force Hc of the soft magnetic material. In order to sufficiently remove distortion and displacement in the metal magnetic particles, it is necessary to heat-treat the soft magnetic material at a high temperature, for example, 400° C. or more, preferably 600° C. or more, and more preferably 800° C. or more.
However, the heat resistance of an insulation coating of a commonly used iron powder with insulation coating is as low as about 400° C., and thus the insulation of the insulation coating is lost by heat-treating the soft magnetic material at a high temperature. Therefore, there is the problem that when the hysteresis loss is decreased, the electric resistivity ρ of the soft magnetic material is decreased to increase the eddy-current loss. In particular, electric equipment has been recently required to have a smaller size, higher efficiency, and higher output, and electric equipment is required to be used in a high-frequency region in order to satisfy these requirements. An increase in the eddy-current loss in a high-frequency region interferes with a decrease in size and increases in efficiency and output of electric equipment.
Therefore, the heat resistance of a soft magnetic material has been conventionally improved by forming an insulation coating composed of silicone of the composition formula (R2SiO)n on the surface of a metal magnetic particle. Silicone has excellent insulation and heat resistance and can maintain insulation and heat resistance as a silica amorphous material (Si—Ox)n even when decomposed by heat treatment at a high temperature. Therefore, when an insulation coating composed of silicone is formed, the insulation of an insulation coating can be suppressed from deteriorating by heat treatment of a soft magnetic material at a high temperature of about 550° C., thereby suppressing an increase in the eddy-current loss of the soft magnetic material. Since silicone has excellent deformation followingness and has the function as a lubricant, a soft magnetic material having an insulation coating composed of silicone is advantageous in that the moldability is excellent, and the insulation coating is not easily broken during molding.
A technique for forming an insulation coating composed of silicone on the surface of a metal magnetic particle is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-254522 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2003-303711 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2004-143554 (Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-254522    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-303711    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-143554